题目内容：

1.__ They provide him wood and other products; they give him shade; and they help to prevent drought and floods.

2.___  In his eagerness to draw quick profit from the trees, he has cut them down in large numbers.

Two thousand years ago, a rich and powerful country cut down its trees to build warships, with which to gain itself an empire. It gained the empire but, without its trees, its soil became bare and poor. When the empire fell to pieces, the country found itself faced by floods and starvation.

Even where a government realizes the importance of a plentiful supply of trees, it is difficult sometimes to make the people realize this. 3.__ So unless the government has a good system of control, or can educate the people, the forests slowly disappear.

4.___ The results are even more serious: for where there are trees, their roots break the soil up, allowing the rain to sink in, and also hold the soil. 5.___ But where there are no trees, the rainfalls on hard ground and flows away on the surface, and this causes floods and the rain carries away the rich topsoil in which crops grow. When all the topsoil is gone, nothing remains but worthless desert．

A. This does not only mean that there will be fewer trees.

B. Trees are useful to man in three very important ways.

C. Unfortunately, in many parts of the world, man has not realized the third of these services is the most important.

D. This prevent the soil from being washed away.

E. If there were no more trees, something terrible would happen.

F. However, a lot of trees are being cut down.

G. They cut down the trees but are too careless to plant and look after new trees.